1. Field of the Invention
The present invention generally relates to a grid array module, and more particularly to a land grid array (LGA) module and a method for forming the same.
2. Description of the Related Art
Traditionally, electronic components, or modules, have been connected to circuit cards (or printed wiring boards (PWB)), by solder, either by pins inserted into plated through holes, or by direct solder attached to the PWB surface. The attachment of an electronic module to a PWB is generally referred to as a xe2x80x9csecond packaging levelxe2x80x9d.
With the increasing complexity of PWBs and their components, reworking modules (e.g., removing modules defective or otherwise from a xe2x80x9ccardxe2x80x9d and replacing the module) has become increasingly necessary, such that module reworkability is now an extremely important design criterion.
Reworking of soldered modules on location is difficult and expensive. Typically, a special apparatus is required for heating a selected module to a temperature beyond the melting point of the solder joints, without disturbing adjacent components.
In response to the demands of component rework, the Land Grid Array (LGA) concept has been developed. In LGA technology, contacts on the module are mechanically held against mating pads on the card, generally augmented by an array of springs functioning as an interposer. The spring array provides the necessary contact force to each module and PWB contact, while providing mechanical compliance to absorb vertical tolerances. The minimum allowable contact force is determined by the properties of the contact force to each module and the anticipated environmental exposure. Typical values are substantially within a range of about 50 grams-force to about 150 grams-force (0.49-1.47 Newtons per contact).
The LGA thus replaces a soldered interconnect array with an array of mechanical pressure contacts, which may be readily separated for module rework.
The LGA has the additional advantage (in addition to easier component rework) that thermal mismatch strains between module and PWB may be absorbed by contact sliding, or in sideways deformation of the interposer contact springs. However, the amelioration of thermal stress is replaced by the introduction of high mechanical forces, which must remain on the module and PWB throughout the product lifetime, and which can potentially compromise the structural integrity of this module.
Thus, land grid array socketing can subject a substrate or the like to a very high level of loading force (e.g., due to a heat sink pushing on a cooling cap affixed to the substrate through a xe2x80x9cseal bandxe2x80x9d) from the array of spring contacts. The loading is typically balanced by mechanical socket forces, which are distributed along the underside of the substrate.
Hence, the substrate undergoes essentially peripheral loading on the top and a distributed loading on the bottom. This imbalance can produce a substantial upward camber and mechanical bending stresses which remain as long as the module remains attached.
As a result, such unbalanced loading can break the substrate catastrophically (especially if made of a weaker material such as glass-ceramic or the like), can reduce the force on central contacts, thus compromising or destroying electrical performance, can squeeze out thermal paste used to thermally connect the chip to the cap, to a degree that it cannot recover during module rework, can squash chip-to-substrate interconnections, especially if not protected with an underfill. Further, the die may be fractured or the seal band damaged or broken.
The conventional methods and designs have attempted to solve the above problems by providing a thicker substrate (e.g., making the substrate have a thickness within a range of about 2 mm to about 8 mm depending on the application involved) and/or a more rigid substrate (e.g., by using stiffer materials, reinforcement members, or the like), but at the expense of performance, space and complexity of manufacturing.
FIG. 1 illustrates an example of a conventional multi-chip LGA module 1. The module 1 includes a substrate 2, which mounts one or more chips or discrete electronic components 3, and a cap 4. The cap 4 serves to mechanically protect the chip, and to provide a heat transfer path from the back of the chip 3 to the external cooling environment. To enhance heat transfer, a highly thermally conductive material 5, such as a paste containing ceramic, metal and/or metal oxide particles or the like, is typically placed between the back of the chip 3 and the cap 4. The cap 4 is attached to the substrate 2 along a peripheral band, or xe2x80x9cpicture framexe2x80x9d-like structure, by a thin layer of adhesive 6 (e.g., a so-called xe2x80x9cseal bandxe2x80x9d). Preferably, the adhesive layer 6 forming the seal band has a thickness substantially within a range of about 10 xcexcm to about 100 xcexcm.
The substrate 2 is attached on the top to the chips or discrete devices 3 by an array of solder joints 7, which may be encapsulated with an underfill material 9 such as silica-filled epoxy or the like. Alternatively, the chips 3 may be back bonded and wire bonded to the substrate 2 (not shown). The bottom of the substrate 2 contains an array of metallized pads 8 which serve to subsequently interconnect the module to the printed wiring board 10. Thus, the module is formed.
The module 1 is clamped to the board 10 by a plate 11 (which may double as a heat sink, and which may have fins 15, to enhance heat transfer) or the like, which is attached to posts 12, which protrude from the board 10. The plate 11 may be attached to the posts 12 by screws or another suitable fastening mechanism, and which may be augmented by springs (not shown).
Alternatively, as shown in FIG. 2, the cover plate 11 (e.g., heat sink) may be shaped so as to attach directly to the board 10, thereby making the posts 12 unnecessary in this design. A soft medium (not shown), such as metal-filled grease, may be placed between the plate 11 and cap 4 to enhance heat transfer.
The substrate pads 8 are connected to mating pads 18, positioned on the surface of the PWB 10, generally through a spring carrier, or interposer, 19 containing LGA contacts 20, which is usually clamped to the board 10 together with the module 1. It is noted that while just two mating pads 18 are shown in FIG. 1, such pads 18 are provided along the entire underside of the substrate 2.
Referring now to FIG. 3, potentially damaging stress, shown by arrows 20 and 21, is imparted to the substrate 2 when the module 1 is attached, due to actuation forces (e.g., from the cover plate 11 (not shown in FIG. 3) and heat sink fins 15 (not shown) or the like) reaching the substrate 2, through the seal band 6.
Since the reaction force on the substrate 2 is not collinear with the seal band 6, but because the reaction force arises from the pads 8, the reaction force is distributed on the bottom surface of the substrate 2. This imbalance leads to upward flexure of the substrate 2, as shown by arrows 21.
Substrate flexure can cause a number of fatal problems, either immediately on actuation, or, even worse, over time. These include failure of the substrate 2 itself (e.g., either catastrophic fracture, internal line tearing, or surface via-via cracks), fracture of the chips 3, excessive squeeze-out of the heat transfer medium 5, delamination of the chip/substrate underfill 9, or rupture of the seal band 6. Clearly, as substrate size increases, or as its thickness decreases, the substrate""s tendency to flex increases.
Minimizing substrate flexure under actuation loading thus becomes a critical factor in the performance and reliability of any LGA.
As mentioned above, several obvious steps have been attempted to remedy the above problems such as by providing for a thicker substrate, a rigid fill material between chip 3 and cap 4, and a very stiff seal band material. However, using any of these generally compromises some other attribute of the module, such as heat transfer or reworkability of the LGA assembly.
It is noted that in another conventional structure (disclosed in U.S. Pat. Nos. 5,757,620 and 5,819,402), directed to customizing a thermal cooling area by providing a different thermal coefficient thermal fluid or paste or compound for each chip in a multi-chip module (MCM), so that each chip can be cooled within its specific specifications, includes a plurality of chips positioned on a substrate. A heat sink or cap is attached to the substrate, and includes a plurality of extensions or partitions that form uniform cavities or blind holes, each for receiving a chip. Thermally conductive paste or grease typically fills the uniform cavity, and more specifically is provided between an upper surface of the chip and a lower surface of the heat sink or cap. The thermally conductive paste or grease provides the heat or thermal transfer path from the chip to the heat sink or cap.
However, such a structure would suffer from many of the same problems above, since the partitions or extensions are not for limiting an amount of flexing of the substrate during actuation. Specifically, unbalanced loading on the substrate may break the substrate and/or damage the chips positioned in the cavities, squeeze out the thermal paste used to thermally connect the chip to the cap, potentially squash chip-to-substrate interconnections, damage a seal band between the heat sink or cap and the substrate, and the like.
In view of the foregoing and other problems of the conventional methods and structures, it is a purpose of the present invention to provide a method and module design which protects a module from damage.
Another purpose is to provide a method and structure in which substrate flexure is minimized (or preferably eliminated) when under actuation loading while simultaneously maintaining module attributes and characteristics such as heat transfer and/or reworkability characteristics of the structure.
In one aspect of the present invention, an integrated circuit module includes a substrate which mounts at least one chip or discrete electronic component thereon, and a cap for covering the substrate, and including at least one protrusion coupled to the cap for limiting the amount of flexing of the substrate during actuation.
In another aspect of the present invention, a land grid assembly (LGA) includes a module having a substrate and a cap, the cap includes at least one protrusion for limiting the amount of flexing of the substrate during actuation, the at least one protrusion extending to a predetermined distance above a surface of the substrate when the module is sealed.
In yet another aspect of the invention, a method of forming a land grid assembly (LGA) module, the method including steps of preparing a cap including sealing legs respectively extending along an outer periphery (e.g., an xe2x80x9couter peripheral picture frame extensionxe2x80x9d) of a first surface of the cap, and at least one integrally formed protrusion on the first surface of the cap intermediate the outer periphery, and joining a substrate to the outer periphery to form a sealed module, the at least one protrusion extending to a predetermined distance above a surface of the substrate when the module is sealed, wherein during a load condition on the substrate, the at least one protrusion suppresses an amount of flexing of the substrate.
With the unique and unobvious aspects of the present invention, a better solution to the problems of the conventional methods and designs is provided, in that the present invention avoids thicker substrates, as in the conventional systems and methods, which increases product cost and which could impact electrical performance.
Further, the present invention avoids use of more rigid substrate materials (e.g., such as alumina ceramic or the like) which may have inferior dielectric properties.
Specifically, the present invention utilizes one or more features which are employed to reduce substrate bowing under LGA socketing. Such a feature includes forming at least one protrusion on the inside of the cap structure which acts as solid stop(s) to limit substrate deformation. The at least one protrusion preferably extends to a predetermined, zero or small distance above the substrate, and may be rounded or capped with a soft material, such as an elastomer, to avoid contacting damage to the substrate. In some cases, the protrusions may be attached permanently to the substrate, such as by soldering. The protrusions may be discrete posts strategically located around the cap, or may be formed as walls.
With such features and others of the present invention, a method and module design are provided which protect a module from damage. Further, the method and structure of the invention minimizes and suppresses substrate flexure when under LGA actuation loading while simultaneously maintaining module attributes and characteristics such as heat transfer and/or reworkability characteristics of the structure.
Furthermore, the present invention is advantageous in harsh environments having severe mechanical shock and vibration characteristics and/or severe thermal characteristics. Specifically, the cap may undergo loading (and flex), thereby imparting load to the substrate. With the unique and unobvious combination of elements, the present invention advantageously protects the substrate in such environments.